Sugar protons stereochemistry

As shown in the biosynthesis of granaticin, a hydride shift occurs intramolecularly. This process is mediated by an enzyme-bond pyridine nucleotide. A concerted abstraction of H-4 as a hydride in la and a C-5 deprotonation in 2a leads to the 4,5-enol ether 3a. The reduced form of the pyridine nucleotide transfers the hydride to C-6, simultaneously releasing a hydroxide to give 4a. Final tautomerization yields the dTDP-4-keto-6-deoxy-sugar in v-xylo configuration 4a. In other enzymes of the oxidoreductase type, the active site may show a different configuration. Thus, the intermediate 3a can be protonated from above at C-5 to yield the l-arabino isomer of 4a [2]. The stereochemistry of this mechanism was demonstrated by double labelling (cf. l-4b series), and as a net result proved a suprafacial 4—>6 hydride shift. 

To the first subgroup belong pauridianthoside (26), to which pauridianthine (7) and pauridianthinol (9) are obviously related, and isopauridianthoside (27). Acetylation of the sugar moiety is necessary to achieve separation of the two epimers, of which the UV, IR, and mass spectra are identical (36,37). Their H-NMR spectra, however, are substantially different, the glucosidic protons as well as the terpenic ones showing very specific chemical shifts and coupling constants, depending on the stereochemistry of the chiral centers (Table III). 

Table 5.5 Sugar stereochemistry at Cl and C2 resulting from protonation of glycals...

The structure of the uridyl peptide antibiotics was elucidated through extensive NMR studies. These studies assigned the cis geometry of the exocyclic enamide double bond based on an NOE (nuclear Overhauser effect) signal between the vinylic proton and the methylene protons of the amino sugar [271, 272], Boojamra et al. established the absolute stereochemistry of 2,3-diaminobutyric acid as (25,35) [273], while Bugg and coworkers demonstrated the remarkable stability of the enamide present in the uridyl peptide antibiotics [274],... 

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